Questions tagged [speed-of-light]

The speed of light is a fundamental universal constant that marks the maximum speed at which energy and information can propagate. Its value is $299792458\frac{\mathrm{m}}{\mathrm{s}}$.

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Surpassing 70%c of spinning black holes - projection [closed]

And in some cases, the holes are spinning at an extremely rapid pace. The hole in one quasar, Einstein Cross, was spinning about as fast as theoretically possible -- 70 percent the speed of light In ...
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Do objects outside the Hubble sphere violate special relativity?

According to special relativity, objects can move with speed at most c. However, objects outside the Hubble sphere recede from us faster than the speed of light. How can these be reconciled?
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Is Feynman right? Does mass change with speed? [duplicate]

In chapter one of Feynman's famous Lectures on Physics, he writes the following. Mass is found to increase with velocity, but appreciable increases require velocities near that of light. A true law ...
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If a proton travels exactly at 99% of speed of light are the individual velocities of the quarks inside it also exactly 99% of $c$?

If a proton travels exactly at 99% of speed of light are the individual velocities of the quarks inside it also exactly 99% of $c$? So can we think that they all travel linearly as the center of mass ...
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Measuring the speed of light with a black hole

is it possible to use the grabity a black hole has to bend the light back to the observer and measure it that way? also saying i made the light go from the right and bend back coming at my left and ...
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Relativistic ship and base on Earth talking

Let say humans make it possible to travel so close to the speed of light that time dilation for the ship can achieve 100 times the time of a standing still object. So one of that ships starts its ...
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How to prove that time stops at the speed of light using general relativity in a flat spacetime?

I know from special relativity that $t'=t/\sqrt{1-(v/c)^2}$, where t is proper time. I see from this equation that for each second of t, t' grows bigger and eventually reaches infinity as v goes to $...
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How can gravity change the wavelength of a photon without the two peaks or troughs to accelerate one from the other?

How can gravity change the wavelength of a photon without the two peaks or troughs to accelerate one from the other? So is it possible that gravity accelerates light? Can the a certain wavelength ...
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At which fraction of light speed could I expand ten times, and 100 times, my clock's ticking? [closed]

I face this problem in special relativity: for dilating the intervals observed in an at rest, foreign frame of reference by ten times, and also by one hundred times, how much should I reduce light ...
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What exactly would it take to show (hypothetically) that the speed of light (in vacuum) is not constant?

Consider Minkowski space, which is determined by the spacetime line element $$ ds^{2} = -c^{2}dt^{2} + dx^{2} + dy^{2} + dz^{2}. $$ Now suppose we modify this so that the $c$ constant is not a ...
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What is wrong with these ways of measuring one-way speed of light? [duplicate]

I thought of 2 ways of measuring one-way speed of light. Can somebody explain why these won't work please? Create a machine that emits a pulse of light and a sound from far away. Have a clock ready ...
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Calculating the one-direction speed of light [duplicate]

According to Derek Muller from Veritasium, measuring the one-direction speed of light requires synchronised clocks, and that's a problem because moving one of the clocks will slow down its time. In ...
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One-way speed of light determined by measuring Time Dilation in two directions? [duplicate]

I know it’s theoretically impossible to measure the one-way speed of light by measuring the time it takes to pass by two points. The issue is with synchronizing two clocks and Time Dilation that is ...
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If during the resistance of a resistor against an electric current particles are colliding why not say that they produce a lot of gamma rays?

During the collision of particles in the LHC we say lots of gamma rays produce but if during the resistance of a wire or a resistor against an electric current particles are colliding why not say that ...
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Why would FTL travel break causality, as oppose to the relative time reconciling as in the Twin Paradox? [duplicate]

I have heard that if faster than light travel were ever made possible, such as by an Alcubierre drive, the user would be taken backwards in time, violating causality. This is odd to me, as it already ...
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Can we really prove that $E_0 = mc^2$ for a photon? [duplicate]

Let $E$ be the totale energy of a particle, $E_0$ the energy when its speed is nil, $m$ its rest mass, $\vec{p}$ the momentum vector, $\vec{v}$ the velocity vector, $c$ the speed of light, $\gamma$ a ...
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Can X-rays travel quicker than visible light in a medium?

I have seen while reading a few books that the refractive index depends on lambda as follows by the Cauchy equation. $${\displaystyle \eta(\lambda )=A+{\frac {B}{\lambda ^{2}}}}$$ From this, the book ...
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What exactly is "dense" in Optical Density?

My book states that: When passing from one medium to another, if light slows down, the second medium is said to be optically denser than the first medium, and if light speeds up, then the second ...
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Does anything "break" in physics if you allow for instant (faster-than-light) communication?

Assume that two entities, Alice and Bob, can communicate thoughts literally instantly. If Bob thinks "Alice, I want you to jump", then Alice (many, many light-years away) will know that ...
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Time and speed of light in Relativity

Time running slower near a massive object, but the speed of light does not really change near a massive object, according to Relativity - it just curves. Is not time directly related to the speed of ...
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How do we know that the gluon travels at $c$? [duplicate]

How do we know gluons travel at the speed of light? can this be derived or is it just an assumption based on the similarity to photons?
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Time differential between two signals sent at two different near-relativistic speeds

There is something I don't quite get about relativistic velocities, which I am hoping to clarify with this question. Suppose you have an emitter and receiver both located somewhere far away from one ...
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Can this experiment successfully be used to find out one-way speed of light? [duplicate]

As with almost every question on this topic, this question is also inspired by the Vertiasium video Why No One Has Measured The Speed Of Light. This experiment tries to measure the one way speed of ...
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A question about relativity and time dilation from a lay-person

Observer A is in a spacecraft flying toward earth Observer B is standing on earth A is traveling at near light-speed and will reach the earth in 1 week. When B looks at A's clock, he sees it moving ...
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What happens to the time period when refraction occurs? [duplicate]

I was studying light and had a doubt that when refraction occurs (rarer to denser just assuming) then frequency remains same but lambda (wavelength) and velocity decrease. But I cant quite figure out ...
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Could the speed of causality be (significantly) faster than $c$?

The other day my son (13) asked me whether it was possible that light went very slightly slower than our best measured $c$, and at the same time had a very tiny mass, but we aren't able to measure ...
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Is light's momentum $0$?

From Paul Dirac's extended mass-energy equivalence equation, we know that, E2 = (mc2)2 + (pc)2 We also know that E = mc2 So, if we write mc instead of ...
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How long would it take to build a wormhole? [closed]

If it were possible to build wormholes, would the process of building them also be faster than light as seen outside the wormhole, or would it take at least as much time as it would light to traverse ...
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In an expanding universe, can two people communicating to each other about their cosmological horizons get around their horizon limit?

I want to pose a preamble question that I will answer first to build up to the main scenario. Then I will pose the main question. The main question concerns the special case of an expanding universe ...
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Has the Michelson-Morley experiment been done in space?

Has the Michelson-Morley experiment been done in space? I mean that exact experiment I realize GPS and similar technologies are doing a similar thing. Are any measuring the literal exact same thing.
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Explanation for invariance of $c$ / Lorentz transformations? [closed]

First of all, I want to make it clear that I'm not proposing here that I have "discovered" something(I'm just trying to save myself here from accusations about being a relativity denier or ...
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Self-coupling of gravity and gravitation escaping a black hole - contradiction?

The field equations are non-linear, that can be interpreted as gravity is coupling with itself, see for example here: Non-linearity and self-coupling of gravity I'm trying to understand what that ...
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How would the following image look like, if we didn't use $ct$ for time?

I just wonder how spacetime would look like if we didn't use $ct$ for $t$ and we just used $t$ instead? I guess the $t$-axis would just scale. Would that mean that, the hyperbolas would be very hard ...
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Energy-momentum tensor of a perfect fluid flowing at the speed of light?

The general energy-momentum tensor for a fluid is given by (working with $c=1$ convention) $$\begin{equation}\label{T}\tag{1}T^{\mu\nu}=\left(\rho+p\right)U^\mu U^\nu+p g^{\mu\nu}\end{equation}$$ with ...
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Special Relativity: If speed of light is constant, then why measured lengths and times aren't changing in the same way? [duplicate]

Special relativity postulate says that speed of light is constant for all inertial observers, meaning that measures of time and distance must accommodate in a way to show that speed of light is the ...
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Relation between time taken by light to pass through a glass slab and the refractive index

I came across the following question as an example in my book: A glass slab has its left half of refractive index $\mu_1$ and right half of refractive index $3\mu_1$. What is the effective refractive ...
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Speed of light and the equivalence principle

In 1913 Albert Einstein wrote: "I arrived at the result that the speed of light is not to to be regarded as independent of the gravitational potential. Thus the principle of the constancy of the ...
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Is our understanding of relativity / the speed of light / anthropocentric due to our limited 5 senses? [closed]

Imagine we were highly intelligent but blind bats, with fantastic sonar that lets us 'see' sound. To us, nothing could travel faster than the speed of sound, because we would only see a sonic boom as ...
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When looking through a telescope at the moon, do I see it at a more recent time point than with my naked eye?

I’m not physics-minded so sorry for the poorly worded question. If I look at the Moon from Earth, my understanding is I am seeing the Moon from around 1 second in the past. But if I also look at it ...
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Lorentz vs Einstein

(1) Lorentz believed in the aether which is known for being the place that all velocities should be measured with respect to, Did lorentz believe that the speed of light is invariant to all observer'...
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Is the speed of signal transport via electricity as fast as light? [duplicate]

Let us assume a time synchronization system that comprises a sender and a receiver. The sender generates and sends an encoded signal which presents the current time to the receiver periodically, and ...
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2 votes
3 answers
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Perception of light speed when traveling between two light sources

I'm struggling with a fundamental understanding of time dilation and special relativity (if I'm correct). Many online sources explain it as the following: With the ship moving away from the lightbeam ...
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Can light exist if it never interacts with anything? [closed]

I've heard some physicists mention that photons do not experience time. From the perspective of the photon, it begins and ends instantaneously. With all the photons shooting out into empty space, it ...
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Under PPN Formalism, would the gravity of a mass double as its velocity approaches $c$?

I'm looking at the answer Michael gave to my question. Does this imply that under parameterized post-Newtonian formalism, under the $β1$ factor for kinetic energy, the gravity of a mass doubles as it ...
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Why should the speed of light necessarily be the same for all observers in train thought experiment?

I am learning about train thought experiment that explains relativity of simultaneity , but this picture confuses my mind. It seems to me even if the speed of light is not the same for all observers,...
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Why does the universality of the speed of light prove there is no ether?

So I definitely don’t think there is an ether for starters. I am reading electrodynamics by Griffith and he states that Einstein’s second postulate which says “The speed of light in vacuum is the same ...
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Special relativity time dilation [duplicate]

According to special relativity time should stop for a particle moving at the speed of light, doesn't that mean that the particle stops moving as well? Then it is not going at the speed of light ...
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Bluetooth Latency

Considering Bluetooth uses EMR, and EMR requires x amount of time to travel any distance. Does this mean the farther my Bluetooth headphones get from my phone, the gaps between each byte of music ...
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Does the relative speed of light, or just the frequency of light, change near a planet?

I have read this this question that describes the red shift in frequency of light near a large planet. I'm wondering if the light will actually reach me slower. Imagine that we have two lasers 1,000,...
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What will be the path of light coming from torch that is lit vertically in a train that is moving at the speed of light compared to the ground?

Assume a train is moving at the speed of light and a person is sitting in the train with a torch. Say the person lit the torch vertically upward comparing to the ground of the train then after the ...
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